Industrial processes that utilize liquid media most often employ solids-liquid separation techniques. In the case of aqueous systems, flocculants are often used to improve the separation process. These processes are practiced in diverse industries such as in the separation of mineral solids from aqueous systems, in the production of pulp and paper and for the treatment of paper wastes as well as for the treatment of industrial and municipal wastes. Currently flocculants are manufactured and sold either as solid powder forms which are difficult to dissolve, or as liquid forms which are easier to handle and use. Liquid forms include water-in-oil emulsions of water-soluble polymers. These have been used for many decades in many diverse industrial applications. However, these product forms suffer from several drawbacks. One of the problems of using the water-in-oil product form occurs when low temperatures (below the freezing point of the emulsion) are encountered. Often, the water-in-oil elusion will invert during the thawing process forming insoluble gels which renders the product unusable. Aqueous solutions of water-soluble polymers also tend to freeze when exposed to low temperatures making them unsuitable for adverse climates. The water-in-oil emulsion forms also suffer from the need to predilute in an aqueous medium prior to use thus adding to the cost for storage tanks and specialized dissolution equipment. When the process is a Bayer process, the pre-dilution step results in an additional problem since it adds water to the process necessitating the addition of additional caustic soda in order to maintain the alkalinity of the system.
There is therefore a need for improved flocculant product forms which can be used in industrial processes to overcome these shortcomings.
The Bayer process is almost universally used to manufacture alumina from bauxite. In this process, raw bauxite ore is first heated with caustic soda solution at temperatures in the range of 140 to 250° C. This results in the dissolution (digestion) of most of the aluminum-bearing minerals, especially the alumina trihydrate AI(OH)3 (gibbsite) and alumina monohydrate boehmite, to give a supersaturated solution of sodium aluminate (pregnant liquor). Resulting concentrations of dissolved materials are very high, with sodium hydroxide concentrations being greater than 150 grams/liter and dissolved alumina being greater than 120 g/l. Any undissolved solids, usually oxides of iron which are known as red muds, are then physically separated from the aluminate solution. Typically a polymeric flocculant is used to enhance the settling and removal of the fine solid particles. Residual suspended solids are removed by a filtration step. The filtered clear solution or liquor is cooled and seeded with alumina trihydrate to precipitate a portion of the dissolved alumina. After alumina precipitation, this depleted or spent liquor is reheated and reused to dissolve more fresh bauxite.
The clarified sodium aluminate liquor is seeded with alumina trihydrate crystals to induce precipitation of alumina in the form of alumina trihydrate, AI(OH)3. The alumina trihydrate particles or crystals are then separated from the concentrated caustic liquor. The alumina trihydrate crystals are generally separated from the liquor in which they are formed by settling and/or filtration. Coarse particles settle easily, but fine particles settle slowly resulting in yield losses. Fine particles can also blind the filters. The fine particles of alumina trihydrate which do not settle easily, are most often recycled back to digestion with the spent liquor. The un-recovered alumina trihydrate is then redigested and reprecipitated in a second cycle through the Bayer process, unnecessarily expending energy and reducing the alumina extraction capacity of the spent liquor. It is therefore highly desirable to settle as much of the trihydrate as possible so as to limit the adverse consequences of these problems.
Canadian Patent No. 825,234, October 1969, uses dextran, dextran sulfate and combinations therewith containing anionic salts to improve the flocculation and filtration of alumina trihydrate from alkaline solutions thereof. U.S. Pat. No. 5,041,269, August 1991, Moody et al., uses a flocculant for the recovery of alumina trihydrate crystals comprising a combination of dextran, or certain other polysaccharides, together with an anionic flocculant polymer including acrylic monomer. Dextran has however proved to be a poor flocculant for trihydrate crystals resulting in poor supernatant clarities.
U.S. Pat. No. 4,767,540 describes the use of hydroxamated polymers for flocculating suspended solids in the Bayer process. Australian patent application AU-B-46114/93 describes the use of certain hydroxamated polymers for the clarification of hydrate solids in the Bayer process.
U.S. Pat. No. 6,608,137 describe water-in-oil emulsions of hydroxamated polymers. These polymers must firstly be dissolved and pre-diluted in an aqueous medium (often a Bayer process liquor) before they may be added to the Bayer process liquor to be settled/clarified.
Thus it is an objective of this invention to provide new high performance compositions of matter, water-in-oil-in-water dispersions of water-soluble polymers, which can be added directly into industrial process streams such as Bayer process steams without predilution thus eliminating the need for expensive storage vessels and associated pumping and dilution equipment. The water-in-oil-in-water emulsions of a hydroxamated polymer of the current invention also exhibit enhanced storage stability over prior art solution and water-in-oil emulsion polymers particularly when subject to extremes of low temperature.
It is also an object of the present invention to provide a more effective Bayer process wherein flocculation, settling, clarification and separation of Bayer process solids, including alumina trihydrate and red mud solids from the process streams is improved by adding to the process stream a water-in-oil-in-water emulsion of a hydroxamated polymer.
These and other objects of the present invention are described in detail below.